1. Field of the Invention
The present invention relates to an organic electroluminescent display device, and more particularly, to a dual plate organic electroluminescent device that includes a first substrate having a thin film transistor array unit, the thin film transistors arranged in a way that prevents point defects, such as dark spots, from occurring in the organic electroluminescent display device.
2. Discussion of the Related Art
An organic electroluminescent display device includes a cathode electrode injecting electrons, an anode electrode injecting holes, and an organic electroluminescent layer between the two electrodes. An organic electroluminescent diode has a multi-layer structure of organic thin films provided between the anode electrode and the cathode electrode. When a forward current is applied to the organic electroluminescent diode, electron-hole pairs (often referred to as excitons) combine in the organic electroluminescent layer as a result of a P-N junction between the anode electrode, which injection holes, and the cathode electrode, which injects electrons. The electron-hole pairs have a lower energy when combined together than when they were separated. The resultant energy gap between the combined and separated electron-hole pairs is converted into light by an organic electroluminescent element. In other words, the organic electroluminescent layer emits the energy generated due to the recombination of electrons and holes in response to an applied current.
As a result of the above-described principles, the organic electroluminescent device does not need an additional light source as compared with a liquid crystal display device. Moreover, the electroluminescent device is thin, light weight, and is very energy efficient. As a result, the organic electroluminescent device has excellent advantages when displaying images, such as a low power consumption, a high brightness, and a short response time. Because of these advantageous characteristics, the organic electroluminescent device is regarded as a promising candidate for various next-generation consumer electronic appliances, such as mobile communication devices, PDAs (personal digital assistances), camcorders, and palm PCs. Also, because the fabricating of such organic electroluminescent devices is a relatively simple process, it is much cheaper to produce an organic electroluminescent device than a liquid crystal display device.
The organic electroluminescent display devices may be provided in either a passive matrix type arrangement or an active matrix type arrangement. The passive matrix type has a simple structure and fabrication process, but has a high power consumption when compared with the active matrix type. Further, because the display size of passive matrix organic electroluminescent display devices is limited by its structure, the passive matrix type can not easily be adapted to a device that is large in size. Moreover, the passive matrix type has a decreasing aperture ratio as the bus lines increase. On the contrary, the active matrix type organic electroluminescent display devices provide a higher display quality with higher luminosity when compared with the passive matrix type.
FIG. 1 is a schematic cross-sectional view illustrating an active matrix type organic electroluminescent display device according to a related art arrangement. As shown in FIG. 1, an organic electroluminescent display device 10 includes first and second substrates 12 and 28 which are attached to each other by a sealant 26. On the first substrate 12, a plurality of thin film transistors (TFTs) T and array portions 14 are formed. A first electrode (i.e., an anode electrode) 16, an organic luminous layer 18 and a second electrode (i.e., a cathode electrode) 20 are sequentially formed on the array portion 14. At this point, the organic luminous layer 18 emits red (R), green (G) or blue (B) color in each pixel P. In particular, to show color images, organic color luminous patterns are disposed respectively in each pixel P.
As additionally shown in FIG. 1, the second substrate 28, which is attached to the first substrate 12 by the sealant 26, includes a moisture absorbent 22 on the rear surface thereof. The moisture absorbent 22 absorbs the moisture that may exist in the cell gap between the first and second substrates 12 and 28. When disposing the moisture absorbent 22 in the second substrate 28, a portion of the second substrate 28 is etched to form a dent. Thereafter, the powder-type moisture absorbent 22 is disposed into this dent and then a sealing tape 25 is put on the second substrate 28 to fix the powder-type moisture absorbent 22 into the dent.
FIG. 2 is an equivalent circuit diagram illustrating a pixel of the organic electroluminescent display device according to a related art arrangement. As shown in FIG. 2, a gate line 30 is disposed in a transverse direction and a data line 32 is disposed in a longitudinal direction substantially perpendicular to the gate line 30. A switching thin film transistor (switching TFT) TS is disposed in a crossing of the gate and data lines 30 and 32 and a driving thin film transistor (driving TFT) TD is disposed electrically connecting with the switching thin film transistor TS. The driving TFT TD is electrically connected with an organic electroluminescent diode E. A storage capacitor CST is constituted between gate 40 and source 42 of the driving TFT TD. The organic electroluminescent diode E is comprised of a first electrode, an organic luminous layer and a second electrode, as described in FIG. 1. The first electrode of the organic electroluminescent diode E electrically contacts with a drain 46 of the driving TFT TD, the organic luminous layer is disposed on the first electrode, and the second electrode is disposed on the organic luminous layer.
Now, an operation of the organic electroluminescent display device will be explained briefly with reference to FIG. 2. When a gate signal is applied to a gate 36 of the switching TFT TS from the gate line 30, a data current signal flowing via the data line 32 is converted into a voltage signal by the switching TFT TS and then this voltage signal is applied to the gate 40 of the driving TFT TD. Thereafter, the driving TFT TD is operated and then determines current level that flows through the organic electroluminescent diode E. As a result, the organic electroluminescent diode E can display a gray scale between black and white.
The voltage signal is also applied to the storage capacitor CST such that a charge is stored in the storage capacitor CST. The charge stored in the storage capacitor CST maintains the voltage of the voltage signal on the gate 40 of the driving TFT TD. Thus, although the switching TFT TS is turned off, the current level flowing to the organic electroluminescent diode E remains constant until the next voltage signal is applied.
Under different circumstances, the above-mentioned organic electroluminescent display device can have plural driving and switching TFTs. For example, the organic electroluminescent display device can have four TFTs, i.e., a four TFT structure. More particularly, the organic electroluminescent display device can have two parallel-connected switching TFTs and two parallel-connected driving TFTs within one pixel. When the organic electroluminescent display device is provided with two switching TFTs, the stress resulting from the continuously applied DC bias decreases to some extent. Generally, the driving TFT can deteriorate as a result of persistent stress from the applied current. As a result, the operating characteristics of the driving TFT can vary severely. To overcome this problem, two driving TFTs can be provided within the pixel in a parallel connection to each other. This results in a prolonged life span of the driving TFT.
FIG. 3 is an equivalent circuit diagram illustrating one pixel of an organic electroluminescent display device having a four TFT structure. As shown, a gate line 52 is disposed over a substrate 50 in a transverse direction, and a data line 54 is disposed in a longitudinal direction substantially perpendicular to the gate line 52. A first switching TFT TS1 and a second switching TFT TS2 are disposed near a crossing of the gate and data lines 52 and 54. Further, in a pixel defined by the gate and data lines 52 and 54, there are first and second driving TFTs TD1 and TD2 which are electrically connected with the first and second switching TFTs TS1 and TS2. Gates 58 and 64 of the first and second switching TFTs TS1 and TS2 are both connected to the gate line 52. Source 60 of the first switching TFT TS1 is connected to the data line 54. Drain 62 of the first switching TFT TS1 is connected with source 66 of the second switching TFT TS2. The drain 62 and the source 66 are formed together as a monolithic structure of single piece. Gates 70 and 71 of the first and second driving TFTs TD1 and TD2 are also formed together as a monolithic structure of single piece and connected to the drain 67 of the second switching TFT TS2. Drain 72 of the second driving TFT TD2 is connected to both the drain 62 of the first switching TFT TS1 and the source 66 of the second switching TFT TS2. Sources 76 and 74 of the first and second driving TFTs TD1 and TD2 are connected to the power line 56. A storage capacitor CST is formed between the gates 70 and 71 and the sources 76 and 74 of the first and second driving TFTs TD1 and TD2. The drain 78 of the first driving TFT TD1 is connected with an organic electroluminescent diode E.
The above-described organic electroluminescent device having a four TFT structure operates like the organic electroluminescent device having a two TFT structure. However, in the organic electroluminescent device of FIG. 3 having a four TFT structure, the data signal is divided when applied to the parallel-connected TFTs. In particular, the data current signal of the data line 54 can flow through the first and second switching TFTs TS1 and TS2, dividedly. Then, the first and second switching TFTs TS1 and TS2 drive both the first and second driving TFTs TD1 and TD2. Thereafter, a current signal flowing in the power line 56 is delivered to the organic electroluminescent diode E through the first driving TFT TD1, thereby emitting light.
The related art organic electroluminescent display device having the two TFT structure or the four TFT structure frequently results in point defects, such as dark spots, being displayed. Since the organic electroluminescent display device is very thin, for example, often having a thickness of 1000 angstroms, the organic electroluminescent display device may be broken, resulting in these dark spots appearing in the displayed images. Furthermore, such point defects typically arise as a result of malfunctions of the thin film transistors within the pixel. For example, if the switching and driving TFTs improperly operate within the pixel, the pixel having the malfunctioning TFT can be erroneously displayed as a dark spot.